Why can't I start the "From Ashes" DLC?

It doesn't matter what disc you have in the tray. Since this is DLC, it is on your hard drive so you can access when using either disc.

How far into the game are you? IIRC you can access this mission as soon as you get control of the Normandy to fly anywhere in the galaxy. There are a couple of story missions that occur before this point.

If you are late in the game, it is possible that you can only access the mission up to a certain point (I have no idea if this is the case or not). If you start a new game, is it available to you as soon as you can fly wherever you want?

The mission itself takes place on Eden Prime, which is in the Exodus Cluster. Can you fly there and find Eden Prime manually? Maybe it will let you land and start the mission even if you haven't received the e-mail about the mission.

Mass Effect Wiki's article on Priority: Eden Prime does not note any mission or plot point prerequisites. However, it does state that Liara T'Soni is a required squad member. So, I expect the mission may be unavailable until the completion of Priority: Mars.

While this is early enough in the game (and, if I'm not mistaken, still part of the game's opening railroad) that I doubt this is your problem, I figured it might still be worth mentioning.

Another thing that may or may not make a difference: Have you checked both your personal terminals? I believe some messages may only come through on the terminal in your quarters, or only on the terminal next to the Galaxy Map. I could be wrong on this, or this might not be one of those things, but it wouldn't hurt to try.

From Ashes takes place on Eden Prime.
You get alerted about a Cerberus presence and a recovered artifact on Eden Prime via your personal computer, and Eden Prime should show on your galaxy map.

It is accessible after Priority: Mars. It should be available up and until the end of ME3 after the Mars mission.

From Ashes: http://masseffect.wikia.com/wiki/From_Ashes

Eden Prime: http://masseffect.wikia.com/wiki/Eden_Prime